--- trunk/OOPSE/libmdtools/calc_LJ_FF.F90	2003/04/04 22:22:30	460
+++ trunk/OOPSE/libmdtools/calc_LJ_FF.F90	2003/07/15 17:10:50	611
@@ -2,7 +2,7 @@
 !! Corresponds to the force field defined in lj_FF.cpp 
 !! @author Charles F. Vardeman II
 !! @author Matthew Meineke
-!! @version $Id: calc_LJ_FF.F90,v 1.2 2003-04-04 22:22:30 chuckv Exp $, $Date: 2003-04-04 22:22:30 $, $Name: not supported by cvs2svn $, $Revision: 1.2 $
+!! @version $Id: calc_LJ_FF.F90,v 1.7 2003-07-15 17:10:50 gezelter Exp $, $Date: 2003-07-15 17:10:50 $, $Name: not supported by cvs2svn $, $Revision: 1.7 $
 
 module lj
   use definitions
@@ -195,6 +195,7 @@ contains
     real( kind = dp ) :: t6
     real( kind = dp ) :: t12
     real( kind = dp ) :: delta
+    integer :: id1, id2
 
 
     if (rij.lt.LJ_rcut)  then
@@ -219,9 +220,9 @@ contains
        
        dudr = 24.0E0_DP * epsilon * (t6 - 2.0E0_DP*t12) / rij
        
-       drdx = -d(1) / rij
-       drdy = -d(2) / rij
-       drdz = -d(3) / rij
+       drdx = d(1) / rij
+       drdy = d(2) / rij
+       drdz = d(3) / rij
        
        fx = dudr * drdx
        fy = dudr * drdy
@@ -254,16 +255,35 @@ contains
 #endif
        
        if (do_stress) then
-          tau_Temp(1) = tau_Temp(1) + fx * d(1)
-          tau_Temp(2) = tau_Temp(2) + fx * d(2)
-          tau_Temp(3) = tau_Temp(3) + fx * d(3)
-          tau_Temp(4) = tau_Temp(4) + fy * d(1)
-          tau_Temp(5) = tau_Temp(5) + fy * d(2)
-          tau_Temp(6) = tau_Temp(6) + fy * d(3)
-          tau_Temp(7) = tau_Temp(7) + fz * d(1)
-          tau_Temp(8) = tau_Temp(8) + fz * d(2)
-          tau_Temp(9) = tau_Temp(9) + fz * d(3)
-          virial_Temp = virial_Temp + (tau_Temp(1) + tau_Temp(5) + tau_Temp(9))
+
+#ifdef IS_MPI
+          id1 = tagRow(atom1)
+          id2 = tagColumn(atom2)
+#else
+          id1 = atom1
+          id2 = atom2
+#endif
+
+          if (molMembershipList(id1) .ne. molMembershipList(id2)) then
+ 
+             ! because the d vector is the rj - ri vector, and 
+             ! because fx, fy, fz are the force on atom i, we need a
+             ! negative sign here:
+
+             tau_Temp(1) = tau_Temp(1) - d(1) * fx
+             tau_Temp(2) = tau_Temp(2) - d(1) * fy
+             tau_Temp(3) = tau_Temp(3) - d(1) * fz
+             tau_Temp(4) = tau_Temp(4) - d(2) * fx
+             tau_Temp(5) = tau_Temp(5) - d(2) * fy
+             tau_Temp(6) = tau_Temp(6) - d(2) * fz
+             tau_Temp(7) = tau_Temp(7) - d(3) * fx
+             tau_Temp(8) = tau_Temp(8) - d(3) * fy
+             tau_Temp(9) = tau_Temp(9) - d(3) * fz
+             
+             virial_Temp = virial_Temp + &
+                  (tau_Temp(1) + tau_Temp(5) + tau_Temp(9))
+
+          endif
        endif
        
     endif